1. Field of the Invention
The following relates to platens commonly used in printers, and especially for high speed printers such as those used in the data processing industry.
2. Description of the Prior Art
Platens are used in high speed printers to position printing paper within the printer so that it may be marked by print elements. The platen, which receives impacts from the print elements as they strike the paper, must have sufficient rigidity to resist excessive deflectional movement. The platen should further be able to dampen vibrations caused by impact of the print elements. Control of platen deflection and vibration is of great importance in high speed printers. It is also important that a high speed printer platen have very precise concentric alignment between the outer circumferential surface of the platen and the mounting journals on which it rotates to prevent eccentric rotation and vibration.
In the past, platen stiffness and damping has been achieved by encapsulating or otherwise sleeving a rigid core with a resilient material such as rubber. Commonly used styles of platens have a cast, wrought or extruded hard plastic or metal sleeve or core. The core is machined to accept journal shafts concentric with the centerline of the core. It is then sleeved with rubber or other resilient material to dampen vibration induced by printer element impact. The final operation in construction of a platen of this type includes a grinding of the outer cylindrical surface of the rubber sleeve. Current practice in fact frequently involves one rough and one finish grinding operation. This is required to create as precise a cylindrical outer surface as possible. With this type of construction, however, core machining errors can lead to initial misalignment of the journal shafts which, in turn, will cause eccentric rotation and vibration of the platen in the printer. Journal misalignment can also cause inaccurate grinding of the outer surface of the sleeve, as the journals are typically used to support the platen during grinding. Metal cores require expensive material and precision machining operations.
In order to reduce costs, platens have also been constructed with parts of the platen structure molded of plastic material. For example, U.S. Pat. No. 3,164,652 shows a method of manufacturing a smooth cylindrically surfaced platen by molding a hollow molten plastic core within a previously formed rubber sleeve. The sleeve is radially constrained in a mold and a mandrel inserted in radially spaced relation within the sleeve. Molten plastic material is then fed into the space between the mandrel and sleeve and allowed to cure. The resulting platen is comprised of an outer sleeve with a molded inner hollow core. No provision, however, is made to provide rigidity to the platen nor does the platen, after the molding operation, have any journals or support shafts. These must be separately manufactured and assembled to the platen core, creating the possibility of misalignment. Thus, if the platen of this connection were to be used in high speed printers, resulting misalignment of the platen in its final assembled position would produce an unsatisfactory condition.
Another type of platen especially designed for business machines, such as typewriters, is disclosed in U.S. Pat. No. 4,186,162. In construction, a metal or plastic tube is used as a core with concentrically aligned journals attached to plugs or collars fitted into the ends of the tube. The tube is filled with a foam plastic mixture for absorbing noise and is finally covered with a resilient sleeve. As for the method of making the platen, a rod is inserted into a first plug and the plug inserted a preselected distance into one end of the tube. A chemical foam mix is fed into the open end of the tube and the other end is then fitted with a second plug, which also retains the rod. The foam mix is cured to form a noise absorbing chamber. The completed tube is then ready for further fabrication, such as covering of the tube with a resilient sleeve. Use of a platen of this type of construction in a high speed printer could clearly present alignment problems because of the manner in which the axle structure is constructed. Furthermore, since the basic structure of the platen, i.e. metal core, outer rubber sleeve, is the same as the previously discussed platens, accurate journals would have to be precisely machined and assembled and the outer sleeve would require final grinding.
A method of roll construction similar to that of U.S. Pat. No. 4,186,162 is disclosed in U.S. Pat. No. 3,662,446. This patent, however, relates to a composite lightweight roll used in corrosive environments. In constructing the roll, a shaft is concentrically positioned in a cylindrical mold cavity. A foam mixture is poured into the cylindrical mold and cured to form a solid core. The core is removed from the mold and covered with a layer of fiberglass reinforced plastic, which is in turn covered by an elastomeric layer. The cylindrical mold may be eliminated by using a fiberglass sleeve as a mold. After curing the foam mix, the fiberglass sleeve is covered with an elastomeric layer.
Similarly, U.S. Pat. No. 442,603 shows a platen manufactured by concentrically casting a hollow hard rubber core around a shaft. The core is, in turn, covered by a softer rubber layer which is applied by vulcanization or by slipping on a separate sleeve.
Still another typical platen construction is disclosed in In U.S. Pat. No. 3,646,652. Specifically, a substantially rigid core is concentrically positioned within a plastic sleeve. The annular gap between the core and sleeve is then filled with a resilient filler.
Each of the constructions in the three last mentioned patents would, at least, require further fabrication after the molding operation if they were to be used in any equipment requiring high precision. Typically, they would require final grinding of the sleeve and/or precise machining thereof to obtain the necessary accuracy of construction. It is important in high speed printers that the platen has a smooth and non-eccentric cylindrical outer surface and that this surface be maintained during high speed rotation. With platens constructed in accordance with the teachings of the above patents, surface distortion or eccentricity would likely result. Correction by machining of the parts or surface grinding of the sleeve is expensive. To reduce manufacturing costs, it is desirable to eliminate these fabricating steps.